Thermally activated batteries are expendable, one-shot, devices. They are typically utilized in equipment such as missiles, torpedoes, or emergency power units to provide a short term source of electrical power for control circuitry or starter motors until an engine driven electrical generator can be brought on line. Equipment of this type must often sit idle for extended periods prior to being activated. These extended periods of inoperation make the use of more conventional batteries impractical, due to the problem of maintaining a proper electrical charge in the battery while the equipment is inoperative.
A typical thermally activated battery includes one or more thermally activated cells, and a heat source. The thermally activated cell includes an anode, and a cathode separated by an electrolyte which is a solid at room temperature. In its solid state, the electrolyte has a very low electrical conductivity. Because of this low electrical conductivity, the cell is essentially inert when the electrolyte is in its solid state. If the temperature is raised above its melting point, the electrolyte becomes ionically conductive, and the cell is capable of delivering electrical energy.
Modern thermal batteries generally use lithium as an anode because it provides superior performance characteristics. Lithium is light weight and has good conductivity. Its standard potential and electromechanical equivalence are higher than all other metals. By virtue of its light weight and superior electrical potentials, electrical cells using lithium as an anode material provide high voltage and high power density. Such cells are also operable over a wide range of temperatures, and feature superior shelf life. In addition to lithium, it will be recognized by those skilled in the art that many other substances may function as an anode/heat source, such as calcium, sodium, potassium, magnesium, and other elements and compounds possessing light weight and high electromotive potential.
In combination with the lithium anode, a modern thermal battery might utilize a cathode of iron sulfide, separated from the anode by an electrolyte of a lithium chloride-potassium chloride material. When heated to about 750.degree. F. (400.degree. C.), the electrolyte becomes molten and ionically conductive.
In prior thermal batteries, pyrotechnic materials such as a zirconium barium chromate heat paper, or a heat pellet containing fine iron powder and potassium perchlorate were often utilized as heat sources. The heat source was generally activated by a percussion-type primer, or an electrical pulse impressed across an electrical match, sometimes known as a "squib" or a "fuse" within the thermal battery.
These prior heat sources are not entirely satisfactory. They have been known to change volume during operation, particularly under the heavy g-loads experienced in some applications. The rate of reaction was sometimes difficult to control, with excessive temperatures (2000.degree. K.) being generated. Such excessive temperature can cause damage to the battery, and reduce the operating time of the battery. The reactions also created a substantial volume of gas in some instances, leading to concerns that high internal gas pressure might rupture a sealed battery case. In some instances the time required after ignition for the heat source to reach the melting point of the electrolyte was too long for effective use of this type of heat source in thermal batteries powering control or fusing circuits in missiles.
In another previously utilized approach, heat sources were provided by fabricating bimetallic structures of metals known to create "exothermic intermetallic reactions (EIR's)". These bimetallic EIR heat sources were formed by laminating together thin foils of the reactant metals by cold-rolling or explosive welding, or by vapor deposition of one metal onto another. Alternatively, mixtures of metal powders compressed into heat pellets were utilized. U.S. Pat. No. 4,158,084 to prentice describes such EIR based heat sources.
EIR based heat sources are not entirely satisfactory. Some of the bimetallic material combinations required to achieve acceptable heat output at a controlled temperature, and reliable initiation of the exothermic reaction, involve metallic materials which are in relatively short supply and are thus generally too costly to be useful as heat sources in practical thermal batteries. In addition, the processes used to produce previously known EIR type heat sources--cold rolling; explosive welding; vapor deposition; mixture of fine powders--are also costly and dangerous, given the pyrophoric nature of the EIR materials.
In summary, prior heat sources in thermal batteries suffered from one or more of the following problems: slow starting; poor control of heat production rate and maximum temperature; potential for rupture due to creation of gases; unacceptably high material and fabrication costs; and danger of light-off during manufacture due to the pyrophoric nature of the materials involved.
It is an object of my invention, therefore, to provide an improved thermal battery, overcoming one or more of the problems described above. Specific objects of my invention include providing:
1. an improved thermally activated cell; PA1 2. an improved heat source for a thermally activated cell; PA1 3. a heat source capable of providing a fast-start reaction, minimal internal pressure rise, and high heat content at predictable temperatures; PA1 4. a thermal battery that may be produced at low cost; PA1 5. a thermal battery which is smaller in size and weight than prior thermal batteries; and PA1 6. a thermal battery offering extended battery operation once activated.